This course is designed to cover subjects in advanced high school chemistry courses, correlating to the standard topics as established by the American Chemical Society. This course is a precursor to the Advanced Chemistry Coursera course. Areas that are covered include atomic structure, periodic trends, compounds, reactions and stoichiometry, bonding, and thermochemistry.

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Very clear in the explanations provided, it is a complete course to really introduce to basic concepts of chemistry such as stechiometry, thermodynamic, atom structures and reactions.

TM

Jun 21, 2017

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This is a great course! Very well explained! It's managing to keep my attention on 100% and that's not an easy task for anybody. Thank you for sharing your knowledge with us!

À partir de la leçon

Week 3

Now that we know the structure of an atom, we can explore how atoms combine to form either molecular or ionic compounds. Then we will learn the rules of nomenclature that ensure that a compound is named according to IUPAC rules. We will end this unit by looking at quantitative relationships for compounds including the molar mass of and mass percent of an element in a compound.

Enseigné par

Dr. Allison Soult

Lecturer

Dr. Kim Woodrum

Sr. Lecturer

Transcription

In this module, we are going to talk about the nomenclature of ionic compounds. By the end of this module, you should be able to name ionic compounds according to the IUPAC rules. IUPAC stands for the International Union of Pure and Applied Chemistry, and this group is the one that decides the nomenclature rules, determines when a new system is needed when there are new compounds being formed, and they're are also the ones that determine which new synthetic elements have merited the classification as an element and should be added to the periodic table. Ionic compounds are fairly easy to name, particularly when we're dealing with our main group elements. Because we only have one option for the cation charge, one option for the anion charge, we really don't have to worry about including any of that information in the name of the compound. So what we do is, we take our name of our atom, that becomes a cation, and we simply add the ion ending on it. So sodium becomes sodium plus, and we call it a sodium ion. Beryllium becomes the beryllium ion. Nothing really to change. And that's what we do when we are just identifying the name of the ion. Anions, which are nonmetals that are gaining electrons, are going to change the ending of their name from whatever it is to ide. So chlorine becomes chloride, oxygen, oxide, sulphur, sulphide. And we'll actually keep that suffix when we form the name of the actual compound. So now we can look at some example names of ionic compounds. Notice that I have a formula of RbBr, which is rubidium and bromine. So the first part stays the same, we always name the cation first, and then we name the anion. And I changed the ending of the anion from ine of bromine to ide of bromide, and that's all we have to do for the name. For AlF3, we have aluminum and fluorine, so the name becomes aluminum fluoride. Nothing in there about the subscripts, because if we go back to looking at chemical formulas, there's only one way that this compound can form. Because aluminum has a three plus charge, fluoride has a minus one charge, in order for it to be electrically neutral, it has to have AlF3. So we don't need to include any of that subscript information in our name. When we look at polyatomic ions, we actually don't mess with their names at all. We leave them as they are. This is the ammonium ion. The name of this ion, CN minus, is actually the cyanide ion. I'm not actually changing the end of that. I'm not going to change anything about it. Here's another example of a polyatomic ion in a compound. Here we have potassium, which is a monatomic ion, so we have potassium, and Cr2O7 has a two minus charge, and the name of it is dichromate, okay? It already has its name for an ion, and so we simply put those two things together. Now, when we look at transition metals, remember that transition metals can have a variety of charges, and if I just have the name, for example, copper chloride, it doesn't tell me which version of copper I have in the compound. And so what we do is, we put Roman numerals in the name, but we only do that for metals that have more than one possible charge. So all of our transition metals except silver, zinc, and scandium, because they only have one option. So for example, AgCl would be named silver chloride. So it is a transition metal, but I don't need to have a Roman numeral in there, because silver can only be plus one. However, when I look at copper, I know that I can have copper plus one or copper plus two. And so if I'm looking at the compounds for these, or the formulas for these compounds, I have CuCl or CuCl2. So I have to include that Roman numeral in the name, and that Roman numeral tells me the charge on that metal ion. A couple of things to notice. One, these are Roman numerals, so we use I, we don't put the number one in there. Notice there's no space between the name of the element and the opening of the parentheses, and there are no spaces inside the parentheses. However, there is a space after the closing parentheses before we name the anion. So any time we have a transition metal, an element that has more than one possible charge, we always want to include a Roman numeral to indicate the charge. We do not want to include a Roman numeral if there's only one possible charge. So for main group elements or for our elements silver, zinc, and scandium, we'll never need to put a Roman numeral at the end of the name because our anions only have one possible charge. It's only the cations that are affected. So what do you think is the IUPAC name for Fe2O3? So our correct name is Iron (III) oxide. So we know Fe is iron, that's the easy part, and oxygen is O, and we change the ending to ide, so we have iron oxide. The challenging part here is figuring out what the charge is on the iron, and the formula actually gives us that information. So we know iron is a transition metal, we know it's not one of our exceptions, so we know we're going to have to determine the charge on the iron in order to put the Roman numeral in. So what I can do is say, okay, the charge on the iron ion is equal to x. I know that the charge on oxygen is minus 2. So what I can do is say 2 times x, which is my charge on the iron, plus 3 times minus 2, and this must be equal to 0, because this is an ionic compound which is electrically neutral. So now I have 2x minus 6 equals 0, 2x equals 6, so x equals plus 3. And so that tells me that the charge on iron in this particular compound is plus 3. And so I'm going to use the Roman numeral of III in the name. Note that it's the charge on the iron, not the subscript that is included in the name of the compound. Iron (II) oxide would actually have the formula FeO, because if I have iron 2 plus and oxygen 2 minus, when I crisscross down, I end up with Fe2O2. And because ionic compounds, I'm only worried about the ratio between the cations and anions, I can simplify that to FeO. Another thing we see with ionic compounds, and sometimes these are ones that have transition metals, so we include the Roman numeral in their name, sometimes not, it just depends on the particular compound. But they exist as a hydrate, and so hydrate, okay, we see the hydra in there, we're talking about water. And so what happens is, certain molecules have a set number of molecules kind of attached to them. And so when I actually look at the sample, if I were to zoom in on those molecules, I wouldn't just see in this case the cobalt chloride, I would also see six water molecules surrounding that unit of compound. And so what I have here is that I have cobalt (II) chloride. This is my anhydrous version, which means it's dry, okay, the water has been removed, no water. It's CoCl2, I do need the Roman numeral there because it's cobalt (II) chloride. When left out in the atmosphere, whether inside or outside, this cobalt (II) chloride will actually absorb water and forms cobalt (II) hexahydrate. For this particular molecule, it attracts six water molecules for every unit of cobalt chloride. And we looked at an example earlier with the iron nitrate which attracted nine. Every molecule that forms a hydrate has its own characteristic number, and there's no way to really predict that just by looking at the compound and saying, oh, of course, this is going to have x number of water molecules. That's information we get from experimental results. So the only thing I do differently in the naming of this compound is, I'm adding on this part at the end. So here I have my cobalt (II) chloride, which is the same as what I had in the anhydrous version. But with the water, this hexa tells me how many water molecules I have. And so hexa means six and hydrate means added waters. So six added waters on this cobalt (II) chloride unit. And when we show this in the chemical formula, we show it with a dot between the compound, the ionic compound and the water formula. Now, when we name bases, they're really easy to name, because we name them like any another ionic compound. And a base is simply a substance that yields hydroxide ions when dissolved in water. And so our most common example of a base is NaOH or sodium hydroxide. And if I put that in water, what I'm actually going to see in the water are sodium ions and hydroxide ions. So because I have those hydroxide ions produced, I call this a base. But the naming convention is no different than any other ionic compound. Hydroxide is one of our polyatomic ions with OH and a minus one charge. So I don't actually have to do anything to the ending, because it's already a hydroxide. And I can do this with any other of my metals. I can have lithium hydroxide. I can have magnesium hydroxide. I have to use my parentheses, because that's a polyatomic ion and there are two of those. And my name there would just be magnesium hydroxide. So I can do that with all of my bases. In the next module, we're going to look at how we name molecular compounds.